Porphyrinoids, a unique platform for exploring excited-state aromaticity

Kim, Jinseok; Oh, Juwon; Osuka, Atsuhiro; Kim, Dongho

doi:10.1039/d1cs00742d

Cited by 50 publications

(45 citation statements)

References 276 publications

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“…The values obtained were below 0.02, which is the threshold for single-configura- tional character of closed-shell species, 93 justifying our use of single-reference methods. Interestingly, for the cubes C 8 (C 4 ) 12 (17) and C 8 (C 6 ) 12 (18), the extents of delocalization of the radial π-electrons are higher at 43 and 42%, respectively. Based on the CC bond lengths within the hexatriyne segments of 18 (1.229−1.330 Å) as compared to those of 1,3,5-hexatriyne (1.209−1.356 Å), it is furthermore clear that there is some degree of bond length equalization in 18 indicative of enhanced bond delocalization, which may suggest aromaticity.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…Using qualitative theory combined with quantum chemical calculations, we came up with key points that help in discerning between regular 2D-aromaticity, albeit in a 3D molecular structure, and true 3D-aromaticity in three-dimen- (18) (O h symmetric) at the optimal B3LYP/6-311G(d,p) geometries. The symmetry of the orbitals is also specified.…”

Section: ■ Conclusionmentioning

confidence: 99%

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Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

et al. 2022

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Several fully π-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3Daromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored hitherto should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. 3D-aromatic molecules have highly symmetric structures (or nearly so), leading to (at least) triply degenerate molecular orbitals, and for tetrahedral or octahedral molecules, an aromatic closed-shell electronic structure with 6n + 2 electrons. Conversely, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Huckel's 4n + 2 rule for each macrocyclic path, yet their π-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., with π-electron counts different from 6n + 2, and they are related to naphthalene. Finally, we identify tetrahedral and cubic π-conjugated molecules that fulfill the 6n + 2 rule and exhibit significant electron delocalization. Yet, their properties resemble those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite the fact that these molecules show substantial π-electron delocalization, they cannot be classified as true 3D-aromatics.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Section: ■ Conclusionmentioning

confidence: 99%

Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

et al. 2022

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“…The values obtained were below 0.02 which is the threshold for single-configurational character of closed-shell species, 93 justifying our use of single-reference methods. Interestingly, for the cubes C8(C4)12 (17) and C8(C6)12 (18) the extent of delocalization of the radial -electrons are higher at 43% and 42%, respectively. Based on the CC bond lengths within the hexatriyne segments of 18 (1.229 -1.330 Å) as compared to those of 1,3,5-hexatriyne (1.209 -1.356 Å), it is further clear that there is some degree of bond length equalization in 18 indicative of enhanced bond delocalization which may suggest aromaticity.…”

Section: Tetra-tethered 2d-aromatics-in-3dmentioning

confidence: 96%

“…Numerous unconventional forms of aromaticity have been identified experimentally in the last decades; Möbius aromaticity in macrocycles and metallacycles, [1][2][3][4][5][6] all-metal σ-aromaticity in the solid state, 7 aromaticity in electronically excited states, [8][9][10][11][12][13][14][15][16][17][18] and several other forms. [19][20][21][22][23][24] Three-dimensional aromaticity (3D-aromaticity) is an intriguing topic introduced by Aihara in 1978 when he analyzed polyhedral boranes using a Hückel-type molecular orbital theoretical approach.…”

Section: Introductionmentioning

confidence: 99%

Three-Dimensional Fully pi-Conjugated Macrocycles: When 3D-Aromatic and when 2D-Aromatic-in-3D?

Bakouri

Szczepanik

Jorner

et al. 2022

Preprint

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Several fully pi-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3D-aromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored thus far should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. 3D-aromatic molecules have highly symmetric structures (or nearly so), leading to (at least) triply degenerate molecular orbitals, and for molecules with tetrahedral or octahedral structures an aromatic closed-shell electronic structure results with 6n + 2 electrons. On the other hand, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel’s 4n + 2 rule for each individual macrocyclic path, yet, their pi-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. The 3D-macrocyclic molecules, suggested to be 3D-aromatic, are instead similar to naphthalene. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., when their pi-electron counts differ from 6n + 2. Finally, we identify tetrahedral and cubic -conjugated molecules that fulfill the 6n + 2 rule and exhibit significant electron delocalization. Yet, their properties resemble those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite that these tetrahedral and cubic molecules show substantial pi-electron delocalization they cannot be classified as true 3D-aromatics.

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“…The excited‐state (anti)aromaticity is determined with Baird rule, which describes excited‐state (anti)aromaticity with a concept of aromaticity reversal that the S 0 ‐state [4n + 2]π aromaticity (and [4n]π antiaromaticity) based on Hückel rule is inverted into [4n + 2]π antiaromaticity (and [4n]π aromaticity) in the excited states (Figure 1a). 5–7 Baird rule provides crucial insight into the understanding of excited‐state processes and involved photo‐physics and chemistry because the completely reversed aromaticity (or antiaromaticity) serves as a stepping stone for rationalizing energetic stabilization (or destabilization) in the excited states 3,8,9 . In other words, understanding the role of excited‐state aromaticity enables a mechanistic elucidation of organic photochemistry and eventually allows us to modify outcomes of excited‐state phenomena.…”

Section: Introductionmentioning

confidence: 99%

Ligand‐to‐metal charge transfer driven by excited‐state antiaromaticity in metallohexaphyrins

Kim

et al. 2022

Bulletin Korean Chem Soc

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We investigated the effect of excited-state antiaromaticity on the ligand-to-metal charge transfer (LMCT) process of metallohexaphyrins. According to Baird rule, the aromatic metallohexaphyrins in the S 0 state become antiaromatic in the S 1 state. Because of the metal-ligand redox-like nature of LMCT process, the LMCT process can release the antiaromatic nature and its energetic destabilization. The metallohexaphyrins show a correlation between the S 0 -state aromaticity and time constants for the LMCT process, which well demonstrates that the excitedstate antiaromaticity at the Franck-Condon state acts as a driving force for the LMCT process. These findings elucidate the effect of excited-state antiaromaticity on the excited-state behaviors and further provide an insight into the photochemistry of organic molecules. K E Y W O R D S aromaticity, Baird rule, excited-state antiaromaticity, ligand-to-metal charge transfer, metallohexaphyrin

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Porphyrinoids, a unique platform for exploring excited-state aromaticity

Abstract: Recently, Baird (anti)aromaticity has been referred to as a description of excited-state (anti)aromaticity.

Cited by 50 publications

References 276 publications

Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Three-Dimensional Fully π-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Three-Dimensional Fully pi-Conjugated Macrocycles: When 3D-Aromatic and when 2D-Aromatic-in-3D?

Ligand‐to‐metal charge transfer driven by excited‐state antiaromaticity in metallohexaphyrins

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